1. Field of the Invention
The present invention relates generally to lithographic apparatus and methods for their use.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
The illumination system as well as the projection system generally comprise components for directing, shaping or controlling the projection beam of radiation. Generally, the projection system comprises adjustable elements to set the numerical aperture (commonly referred to as the “NA”) of the projection system. For example, an adjustable NA-diaphragm is provided in a pupil of the projection system. The illumination system typically comprises adjustable elements for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution upstream of the mask (in a pupil of the illumination system). A specific setting of σ-outer and σ-inner may be referred to hereinafter as an annular illumination mode. Controlling the spatial intensity distribution at a pupil plane of the illumination system can be done to improve the processing parameters when an image of the illuminated object is projected onto a substrate.
Microchip fabrication involves the control of tolerances of a space or a width between devices and interconnecting lines, or between features, and/or between elements of a feature such as, for example, two edges of a feature. In particular the control of space tolerance of the smallest of such spaces permitted in the fabrication of the device or IC layer is of importance. Said smallest space and/or smallest width is referred to as the critical dimension (“CD”). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners or step-and-scan apparatus, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
With conventional projection lithographic techniques it is well known that an occurrence of a variance in CD for isolated features and dense features may limit the process latitude (i.e., the available depth of focus in combination with the allowed amount of residual error in the dose of exposure of irradiated target portions for a given tolerance on CD). This problem arises because features on the mask (also referred to as reticle) having the same nominal critical dimensions will print differently depending on their pitch on the mask (i.e. the separation between adjacent features) due to pitch dependent diffraction effects. For example, a feature consisting of a line having a particular line width when in isolation, i.e. having a large pitch, will print differently from the same feature having the same line width when together with other lines of the same line width in a dense arrangement on the mask, i.e. having small pitch. Hence, when both dense and isolated features of critical dimension are to be printed simultaneously, a pitch dependent variation of printed CD is observed. This phenomenon is called “iso-dense bias”, and is a particular problem in photolithographic techniques. Iso-dense bias is measured in nanometer and represents an important metric for practical characterization of lithography processes.
Conventional lithographic apparatuses do not directly address the problem of iso-dense bias. Conventionally, it is the responsibility of the users of conventional lithographic apparatuses to attempt to compensate for the iso-dense bias by either changing the apparatus' optical parameters, such as the numerical aperture of the projection lens or the σ-outer and σ-inner settings, or by designing the mask in a such a way that differences in dimensions of printed isolated and dense features are minimized. This last technique may for example involve reticle oversizing and/or Optical Proximity Correction (referred to hereinafter as OPC). For simplicity, reticles provided with patterns to which any kind of compensation for iso-dense bias has been applied, may be referred to hereinafter as OPC-reticles.
Generally, in a high volume manufacturing site different lithographic projection apparatus are to be used for the same lithographic manufacturing process step to insure optimal exploitation of the machines, and consequently (in view of, for example, machine-to-machine differences) a variance and/or errors in CD may occur in the manufacturing process. Generally, the actual pitch dependency of such errors depends on the specific layout of the pattern and the features, the aberration of the projection system of the lithographic apparatus in use, the properties of the radiation sensitive layer on the substrate, and the radiation beam properties such as illumination settings, and the exposure dose of radiation energy. Therefore, given a pattern to be provided by a patterning device, and to be printed using a specific lithographic projection apparatus including a specific radiation source, one can identify data relating to iso-dense bias which are characteristic for that process, when executed on that lithographic system. Said data, and in particular a listing or plot of CD as a function of pitch for a given apparatus, pattern, and process, is referred to hereinafter, for simplicity, as an “iso-dense bias characteristic”. In a situation where different lithographic projection apparatus (of the same type and/or of different types) are to be used for the same lithographic manufacturing process step, there is the problem of mutually matching the corresponding different iso-dense bias characteristics, such as to reduce CD variations occurring in the manufacturing process.
A known technique to match an iso-dense bias characteristic of a machine (for a process whereby an annular illumination mode is used) to an iso-dense bias characteristic of another machine is to change the σ-outer and σ-inner settings, while maintaining the difference between the σ-outer and σ-inner settings (i.e. whilst maintaining the annular ring width of the illumination mode) of one of the two machines. The nominal σ-settings are chosen such as to optimize the process latitude (in particular, the depth of focus and the exposure latitude). Therefore, this approach has the disadvantage that for the machine whereby the σ-settings are changed, the process latitude is becoming smaller and may become too small for practical use.